Chagas' disease is a tropical disease that remains one of the great scourges of mankind. 18-20 million people suffer from this disease and 90 million are at risk. Few cases have been reported in the U.S., but the presence of the parasite in the blood supply is becoming a concern. The effectiveness of the compounds used in the treatment of the chronic form of the disease is uncertain, and the need for more active and less toxic compounds for the treatment of acute and chronic infections has been described. Studies of the basic biochemistry of parasites could provide insights into the physiology of the parasites. Conversely, adequate understanding of the physiology of the parasites could lead to the rational design of new and more effective drugs. The goal of the present project is to contribute to the identification of new opportunities for antiparasitic chemotherapy. Calcium homeostasis in these parasites appears to have some peculiar characteristics not present in the mammalian hosts, such as the presence of an important ATP-driven Ca2+/H+ exchange system associated to acidic vacuoles, named acidocalcisomes. The specific aims of this project are (1) to identify at the ultrastructural level the sites of calcium localization in different trypanosomatids, in particular Trypanosoma cruzi, and T. brucei; (2) to identify at the ultrastructural level the acidocalcisomes; (3) to characterize at the ultrastructural level the presence and distribution of a vacuolar H+-ATPase in these parasites; (4) to characterize at the ultrastructural level the subcellular fractions obtained by different fractionation methods in order to achieve the purification of the acidic vacuoles, and (5) to explore by electron microscopy techniques the presence of acidocalcisomes in other protozoa. Cytochemical and immunocytochemical techniques will be used for that purpose.